Phenology, floral sexuality and breeding system in rambutan (Nephelium lappaceum)
Extensive studies on phenology, floral biology and sexuality, pollination biology, breeding system and fruit set were carried out for three years on two popular Indian cultivars, 'N 18' and 'E 35'. The species is androdioecious with separate male and bisexual individuals. Male trees are absolutely absent in the orchards. Out of 2000 flowers produced in a panicle of cultivar 'N 18', less than 10 flowers were found to be male flowers with functional pollen grains. The bisexual flowers are functionally female as the anthers do not dehisce and shed pollen grains. Detailed examination of stigma for the studies on pollen germination and pollen tube growth using aniline blue fluorescence technique revealed lack of pollen on the stigma surface through open pollination. This contributes to poor fruit set and high rate of fruit drop. The lack of free pollen available for pollination and long period of inactive growth which occurs after anthesis of bisexual flowers clearly shows that fruit set is due to apomixis. High fruit drop attributed to rambutan could possible be due to apomictic mode of fruit development. Foliar application of NAA on unopened flowers in the panicle results in pollen availability by turning some of the functionally female flowers into male flowers with viable pollen. NAA application is highly recommended for good yield and high quality fruits. The stigma remain viable for a day. Honey bees are major pollinators. Nectar is the only reward for the visitors. There is a huge concern for genetic erosion due to selective planting of cultivars in the orchards. Modern cultivars produce a large number of functionally female flowers and a very small proportion of male flowers, or male flowers totally absent. Male trees are not cultivated in the orchards because of unproductivity. Limited resource availability and lack of male trees in the orchards contribute to poor fruit set. There was no marked difference in flowering behavior or floral biology in the two cultivars.
Veliyil George, S. (2019). Phenology, floral sexuality and breeding system in rambutan (Nephelium lappaceum). Acta Hortic. 1241, 239-246
rambutan, underutilized fruit, reproductive biology, pollen-pistil interaction, androdioecy, breeding system